Does a TSH level of 0.8 indicate an overactive or underactive thyroid gland?

TSH:Cant tell without the total T4 or free t4. Low TSH is usually a sign of hyper thyroidism if your thyroid is suppressing TSH production due to over secretion of thyroid hormone. Rarely the brain signal from pituitary can be low leading to what's called "central" hypothyroidism.